The long range goal of these studies is to develop more effective and specific immunosuppression for clinical organ transplantation, obviating requirements for chronic immunosuppression. The proposal focuses on a posttransplant strategy for inducing specific allogeneic unresponsiveness in outbred nonhuman primate kidney transplant recipients. The research is a continuation of previous work which developed a model of kidney allograft tolerance in MHC mismatched rhesus monkeys, using a short course of rabbit antihuman thymocyte globulin (RATG) followed by recipient inoculation with donor bone marrow (DBM). The studies seek to enhance tolerance induction in this model by further defining the DBM population which is active in inducing unresponsiveness and by investigating novel and synergistic immunosuppressive strategies to supplement the basic model. Initial studies have suggested that the tolerance promoting activity of DBM resides in the DR- cell fraction and that the mechanism of unresponsiveness involves a down regulation of antidonor reactive cytotoxic T lymphocyte (CTL). In vitro cell mediated lympholysis studies have demonstrated suppressive activity in a rhesus monkey BM cell subpopulation which has a DR-, CD16+, CD2+, CD8+ phenotype, similar to that expressed by monkey NK cells. The specificity of the suppression in CML suggests a veto cell type mechanism. Initial studies indicate Cyclosporine (CsA) acts as an adjunctive agent to promote tolerance induction in RATG/DBM treated recipients, and Azathioprine acts as a contra-adjunct, impairing the suppressive mechanism. The aims of this proposed studies are to 1) further dissect the DBM cell population relevant to this suppressive mechanism and to establish optimal time dose parameters for DBM. 2) To enhance tolerance induction in this model by investigating limited treatment with CsA, Cyclophosphamide (CYP), a new immunosuppressive agent FK-506, posttransplant TL1, or monoclonal antiCD4 antibody. 3) To investigate the veto cell type mechanism by which CTL activation is suppressed by BM cells. Specifically, we will investigate the activity and frequency of the donor reactive recipient CTLp population in RATG/DBM treated recipients and elucidate the nature of the DBM cell mechanism which suppresses CTL activation. 4) To test the hypothesis that the suppression of allograft rejection in RATG/DBM treated recipients is related to a local intragraft suppressive mechanism involving allograft residence of microchimeric DBM cells which down regulate activation of antidonor CTL. Studies using recombinant DNA transfection techniques are included to determine whether these intragraft suppressive cells are of DBM origin. We are unaware of any study of this kind and expect these investigations will develop a valid preclinical foundation to guide application of the model to clinical transplantation.